how can i start blender 2.8 in ubuntu 18.04
The version you downloaded might not be configured for your distribution. To see if there were some error messages you can open a Terminal window in the folder you extracted blender to by right clicking in the file browser and choosing "Open in Terminal". Then run the program by typing "./blender" (or whatever the program file is called), hit enter and see if there are any error messages.

However I would advise to install Blender from another source that has been tested with your OS instead. There is a PPA for newer versions of Blender maintained by Thomas Schiex: https://launchpad.net/~thomas-schiex/+archive/ubuntu/blender
If you don't mind using the newer development version of Blender (2.81) it might be what you want. You can add a PPA to your system by following the instruction close to the bottom of the linked page or through the Synaptic package manager.